Selecting Vertices on Symmetry Line of Mirror Modifier
A simple code to retrieve vertices that are on the standard (centered X axis) symmetry plane.

This assumes to be without other modifiers before the mirror.

Principle:


Get all edges that have only one linked face (border edges)
Test their distance to the symmetry plane under a limit which should be similar to the merge limit of the mirror modifier


The code:

import bpy
import bmesh
from mathutils import Vector, geometry

obj = bpy.context.object

bm = bmesh.new()
bm.from_mesh( obj.data )

bm.verts.ensure_lookup_table()
bm.edges.ensure_lookup_table()

# If mirrored on self with X axis
center = Vector( (0,0,0) )
axis = Vector( (1,0,0) )

limit = 0.00001

border_verts = set()

for e in [e for e in bm.edges if len(e.link_faces) == 1]:
    if geometry.distance_point_to_plane( e.verts[0].co, center, axis ) &lt; limit:
        border_verts.add( e.verts[0] )
    if geometry.distance_point_to_plane( e.verts[1].co, center, axis ) &lt; limit:
        border_verts.add( e.verts[1] )

print( len( border_verts ) )